Fetal tissues are comprised of many undifferentiated cells that proliferate actively, highly activated cells, nascent vascular endothelial cells, and so on. Although the activity of these cells in fetal tissues is stringently regulated and inhibited as individuals mature, the state of fetal tissues can be considered similar to that of a solid tumor except that the activity is regulated. Therefore, some of the genes expressed specifically or more strongly in fetal tissues (fetal genes) can be genes involved in the phenomena characteristic of solid tumors such as abnormal growth, immortalization, infiltration, metastasis, and angiogenesis. In addition, some diseases other than tumors are also supposed to arise because fetal genes, which are repressed in a normal living body, are abnormally activated. Therefore, genes involved in various diseases such as tumors can be screened by isolating and analyzing fetal genes.
However, there are still few reports on systematic analysis focusing merely on fetal genes from these viewpoints, and at present there is a far from perfect understanding of these gene groups.